jennis,
Mail only supports one Full Name for each account, even if an account has multiple Email Address items (that don't accept full names; I've tried). Unfortunately the Email Address field is still an overly simple, error-prone single line of comma (or space, I think) separated addresses. Instead, it could be a sheet that lets you enter Full Name / Address pairs (at least).
Workaround: use
leon's applescripts. They're still functional with Mail at least up to 10.4.11; haven't tried 'em on Leopard. Even with some minor quirks they've been acceptable for infrequent use.